1. Field of the Invention
The present invention relates to an optical pickup, and an optical disc player, for writing and/or reading an information signal into and/or from a disc-like optical recording medium such as an optical disc and magneto-optic disc.
2. Description of Related Art
Disc-like optical recording media including optical discs and magneto-optic discs have been proposed as information signal recording media, and optical pickups for writing and/or reading information signal into and/or from such disc-like optical recording media, have also been proposed. Such a disc-like optical recording medium comprises a clear substrate made of a clear material such as polycarbonate, and a signal recording layer formed, by coating, on one of the main sides of the clear substrate.
FIG. 1 is an axial-sectional view showing the construction of such a conventional optical pickup. As shown, the optical pickup typically comprises a frame 201, and a semiconductor laser 202 as a light source and a photodetector 207 provided inside the frame 201. A light beam generated by the semiconductor laser 202 travels through a beam splitter 203 and a collimator lens 204, and then it is incident upon an objective lens 205. This objective lens 205 is movably supported by a biaxial actuator 206 supported on a support plate 210 mounted on the frame 201.
The laser beam incident upon the objective lens 205 is focused, by the objective lens 205, onto a signal recording surface of the disc-like optical recording medium. At this time, the laser beam is first incident upon the clear substrate of the disc-like optical recording medium, passes through the clear substrate and focused onto the signal recording surface being the surface of a signal recording layer. The objective lens 205 is moved by the biaxial actuator 206 to always focus the laser beam onto a position on the signal recording surface where the information signal is to be recorded, that is to say, onto a recording track. The recording track is spirally formed on the main side of the disc-like optical recording medium.
In the disc-like optical recording medium, the information signal is written or read at or from the spot of the laser beam focused by the objective lens 205.
The laser beam focused onto the signal recording surface is modulated in amount of light or polarizing direction correspondingly to the information signal recorded there on the signal recording surface, and reflected by the signal recording surface, and returns to the objective lens 205.
The reflected light beam from the signal recording surface travels through the objective lens 205, collimator lens 204 and beam splitter 203, and then it is incident upon a photodetector 207. The photodetector 207 is a light detection element like a photodiode. It detects the reflected laser beam coming from the disc-like optical recording medium through the objective lens 205, and converts it to an electrical signal. The information signal recorded in the disc-like optical recording medium is reproduced based on the electrical signal output from the photodetector 207.
Also, based on the electrical signal delivered from the photodetector 207, there will be generated a focus error signal indicative of a distance between a spot of the light beam focused by the objective lens 205 and the signal recording surface in the direction of the optical axis of the objective lens 205, and a tracking error signal indicative of a distance between the light beam-focused spot and the recording track on the signal recording surface in the radial direction of the disc-like optical recording medium. The biaxial actuator 206 is controlled based on these focus and tracking error signals to move the objective lens 205 until each of the error signals is reduced to zero.
Optical recording media of the above-mentioned disc-like type and having a higher density of information signal recording have been demanded and thus are under research and development for use as auxiliary storage units for computers, and audio and video signal recording media.
For writing and reading information signal into and/or from a disc-like optical recording medium having thus a high capability of recording, the objective lens 205 should be designed to have a larger NA (numerical aperture) and the light source should be made to generate a light beam of a shorter wavelength to reduce the size of a focused spot of the light beam on the disc-like optical recording medium.
With a larger NA of the objective lens 205, however, the allowances for skew of the objective lens 205 in relation to the disc-like optical recording medium, for thickness nonuniformity of the clear substrate in the medium, and for defocusing of the light beam on the medium, will be smaller with the consequence that it is difficult to write and read information signal into the disc-like optical recording medium.
For example, when the objective lens 205 skews in relation to the disc-like optical recording medium, a wave-front aberration will take place in the light beam focused onto the signal recording surface, and have an influence on an RF output which is an electrical signal output from the photodetector 207. In the wave-front aberration, a third-order comatic aberration prevails. The third-order comatic aberration will take place in proportion to the NA of the objective lens 205 to the third power, and also to the skew angle of the disc-like optical recording medium to about the first power. Therefore, the allowance for skew of the objective lens 205 is in inverse proportion to the third power of the NA of the objective lens, so that with a larger NA, the allowance becomes smaller.
Therefore, in the optical pickup, the mounting angle of the biaxial actuator 206 in relation to the frame 201 incorporating the light source is adjusted to keep the objective lens 205 from skewing in relation to the disc-like optical recording medium.
To adjust such a mounting angle, a spherical adjusting mechanism has been proposed which comprises a spherical convexity 208 formed on the bottom of a support plate 210 abutting on the frame 201, and a spherical concavity 209 formed on the top of the frame 201 on which the support plate 210 abuts and in which the spherical convexity 208 is fitted, as shown in FIG. 1. The center of curvature of the spherical convexity 208 is located at a position on the optical axis of the objective lens 205 where the focused spot of the light beam having passed through the objective lens 205 skewed about the center of curvature will be least off a focused spot which would be when the light beam passes through the objective lens 205 not skewed.
When adjusting the mounting angle of the support plate 210 in relation to the frame 201, the support plate 210 is pressed toward the frame 201 under the action of an resilient member like a spring, and turned along the spherical concavity 209 with the spherical convexity 208 kept fitted in the spherical concavity 209. After completion of the adjustment of the mounting angle of the support plate 210 with respect to the frame 201, the support plate 210 is fixed to the frame 201 with a screw or the like.
FIG. 2 is a side elevation showing the construction of another conventional optical pickup. As seen, the frame 201 is ridged at 211 on the top thereof to support the support plate 210 nearly at its center. This is another solution having been proposed by far to adjust the mounting angle of the biaxial actuator 206.
In this second conventional mounting-angle adjusting mechanism, the support plate 210 has a support screw 213 and an adjusting screw 212 provided in one end and the other end, respectively, thereof. The support screw 213 and adjusting screw 212 are driven in the frame 201. Also a compression spring 214 is provided between the head of the support screw 213 and the support plate 210. The compression spring 214 presses the support plate 210 toward the frame 201 as indicated with an arrow a in FIG. 2. The head of the adjusting screw 212 abuts on the support plate 210. By adjusting the driven depth of the adjusting screw 212 in the frame 201, the support plate 210 is turned about the ridge 211 as indicated with an arrow b in FIG. 2 and thus the skew of the support plate 210 in relation to the frame 201 is adjusted.
However, the optical pickup having the above-mentioned spherical adjusting mechanism is disadvantageous in that the thickness of the optical pickup cannot be reduced, that is to say, the optical pickup cannot be designed more compact in the optical-axial direction of the objective lens 205. This is because for a reduced thickness of the optical pickup, the spherical convexity 208 has to be supported at the lateral sides thereof from below and an erroneous radius of curvature, indicated with an arrow r in FIG. 1, of the spherical convexity 208 will result in an erroneous optical-axial direction of the objective lens 205.
Also, the optical pickup having the mounting-angle adjusting mechanism in which the support plate 210 is supported on the ridge 211 is disadvantageous in that the area of light projection in the horizontal direction, namely, in a plane perpendicular to the optical axis of the objective lens 205, will be larger. This is because the frame 201 should have some portions thereof used for receiving the adjusting screw 212 and support screw 213.
Furthermore, the mounting-angle adjusting mechanism has a problem that the angle adjustment can be done only around one axial direction parallel to the ridge 211 and the spacing of this pivot from the objective lens 205 will cause the focused spot of the incoming light beam from the objective lens 205 to be largely off a desired position.